In minimally invasive surgical procedures, access to the body is obtained through one or more natural openings or small percutaneous incisions. Medical devices are inserted through these openings and directed to a region of interest within the body. Direction of the medical devices through the body is facilitated by navigation technology wherein the real-time location of a medical device is measured and virtually superimposed on an image of the region of interest. The image may be a pre-acquired image, or an image obtained in near real-time or real-time using known imaging technology such as X-ray, computed tomography (CT), magnetic resonance (MR), or ultrasound (US).
Conventional navigation technology measures the location and orientation of a remote unit attached to the medical device relative to a reference unit. Patient motion can be taken into account by rigidly mounting the reference unit directly onto the patient. A reference unit attached in this manner is also referred to as a dynamic reference because it moves along with the patient.
The remote unit is typically attached to a proximal end of the medical device, however, the location and orientation of the distal end of the medical device is often of primary importance. It is therefore necessary to determine with a high degree of precision the location and orientation of the distal end of the medical device relative to the remote unit. This determination is generally obtained by calibrating the location of the distal end or tip with respect to the remote unit. Tip calibration is typically performed by placing the tip of the medical device in a fixed location such as a dimple on the reference unit.
The problem is that some instruments do not have a well-defined tip such as aspirators, which have a blunt end or a ring curette that has an annular working end instead of a tip. For calibrating trajectory, one known technique is after calibrating the instrument tip, the user calibrates a “back-tip”. By connecting the instrument's “front tip” with a “back tip”, the trajectory is defined. Again, the problem with calibrating the trajectory, may be the lack of a well defined back-tip.